For conventional laser machining such as drilling and cutting, infrared laser such as CO.sub.2 laser is generally used. But, cutting using the infrared laser has disadvantages that the periphery of a cut portion has heat distortion, thermal property change and limited machining precision because such cutting is made by vaporizing and melting the cutting portion by heat.
Instead, drilling and cutting using UV laser light (ultraviolet laser light) of harmonics of excimer laser and YAG laser are attracting attention these days.
The laser cutting using the UV laser light, which does not use the laser beams as heat energy, is cold cutting to cut bonding by a photochemical reaction in resonance with laser wavelengths. Therefore, the laser cutting is basically ideal, high precision cutting free from an influence of heat and heat distortion. Especially, the excimer laser is considered suitable for drilling and cutting of polymer materials, ceramics and composite materials.
But, when the laser machining system using the UV light is operated with the repetition frequency (oscillating frequency) of pulsed oscillation raised to the neighborhood of the maximum frequency of approximately some hundreds hertz, there is a problem that machining precision is heavily lowered because of an effect of heat which was also caused in machining using the infrared laser. The cause is considered that when the repetition frequency of pulsed oscillation is increased, the UV laser machining also causes the same evaporation of a material being machined as in the infrared machining with the progress of cutting or drilling, and heated molecules of the evaporated material being machined adhere to the surface of the material being machined.
As described above, the prior art using the UV pulse laser light to drill or cut irradiates continuously the UV pulse laser light to a single machining spot until drilling or cutting is completed, and when the repetition frequency of the pulse laser light is increased to improve a machining rate, the same thermal effect as seen in the infrared laser machining occurs to heavily degrade machining precision.
In order to remedy such a drawback, the irradiation of laser light may be paused periodically, but such a method makes it meaningless to improve the machining rate by increasing the repetition frequency.
In view of the circumstances described above, the invention was completed to provide a laser machining system which can make laser machining with high precision while keeping a machining rate as high as possible and suppressing a thermal influence.